1. Field of the Invention
The present invention is in the field of magnetic susceptometers, especially those intended for use in ferromagnetic foreign body (FFB) detection as a safety measure prior to magnetic resonance imaging (MRI).
2. Background Art
Ultra-sensitive magnetic susceptibility measurements are useful in a number of applications, including the measurement of iron concentrations in the liver and the detection of ferromagnetic foreign bodies in the eye and brain, and elsewhere in the body, prior to magnetic resonance imaging. In the magnetic susceptibility measurement, a magnetic field is applied to a specimen, and a magnetic sensor measures the change in magnetic field due to the magnetization induced in the specimen by the applied field. The main challenge in such measurements is not only that the magnetic field response of the sample is small in absolute terms, but that the response may be many orders of magnitude smaller than the magnetic field that is applied to the specimen. Measuring such a small response, in the presence of such a relatively large applied field, is especially difficult in a room-temperature instrument, because temperature fluctuations may distort the geometrical relationship between the magnets or coils that produce the applied magnetic field and the magnetic-field sensors that detect the response of the specimen. This geometrical distortion causes fluctuations in the measured magnetic field, which can mask the desired magnetic field response of the specimen. One way to minimize such temperature drifts is to modulate the distance or spatial relationship between the specimen and the instrument, modulating the magnetic susceptibility response of the specimen at a frequency that is relatively high compared with the typically slow time-scale of the temperature drifts.
An alternative approach in sensitive magnetic susceptibility measurements is to maintain a desired geometrical relationship between the magnetic-field source(s) and the magnetic-field sensors, so as to minimize fluctuations in the measured magnetic field. The required dimensional stability of the sensor unit is determined by the required resolution of the magnetic susceptibility measurement. Certain specific applications, such as detection of ferromagnetic foreign bodies prior to MRI imaging, require resolution of changes in magnetic field that are 107 or even 108 times smaller than the field applied to the specimen. In order to resolve signals 107 times smaller than the applied field, it would be necessary for all the relative dimensions of the sensor unit to remain constant to roughly one part in 107. Some existing magnetic susceptometers, based upon superconducting quantum interference devices (SQUIDs), achieve the required stability by placing the magnetic field source or sources and magnetic field sensor or sensors in a liquid helium bath. With this approach, thermal expansion is not a problem because temperature fluctuations are controlled by the liquid-helium bath, and because the thermal expansion of most materials is essentially frozen out at liquid-helium temperatures. This geometrical stability, and not any intrinsic property of superconductors, may in fact be the single greatest advantage of working at liquid-helium temperatures. Achieving similar stability at room temperature is a significant problem.
Even if the aforementioned problems can be solved, there are additional problems related to the use of magnetic susceptibility measurements in pre-MRI screening for FFBs, including masking of the magnetic susceptibility response of the FFB by magnetic susceptibility signals from tissues in the patient's head, and masking of the magnetic susceptibility response of the FFB by the very large response due to the magnetic susceptibility contrast between the body tissues and the surrounding air. Further, the computer equipment used in interpreting the signals measured by the magnetic susceptibility instrument can add to the cost of pre-MRI screening.